There are quite a few things to check - but at a minimum you should get the following:
total Test A good overall measure of hypogonadism but Free test is better
Free Test (the amount of testosterone that's actually available to your body
Estradiol (The aromatizing version of estrogen. You don't need total E - only this one)
LH (measures the pituitary's ability to send the signal to produce Test.
FSH (if you are concerned about sperm count)